1. Field of the Invention
The invention relates to power converters, and more particularly, to power inverters capable of achieving zero-voltage-switching (ZVS) to improve converter's efficiency and reducing input current ripple to alleviate the electromagnetic interference intensity (EMI).
2. Description of the Related Art
To reduce the size and weight with faster dynamic response, PWM power converters are operated at high frequencies. However, there are several inherent problems, such as the increase of the switching losses and the parasitic oscillation noise. Thus, the operating frequency of PWM converters is limited. To alleviate these problems, soft-switching techniques, zero-voltage-switching (ZVS) and/or zero-current switching, are widely used to minimize the switching losses. Employing phase shift modulation or asymmetrical pulse width modulation scheme, several high frequency zero-voltage full-bridge topologies for high power applications were disclosed.
For example, U.S. Pat. No. 6,466,458, issued to Zhang et al, discloses an asymmetrical full bridge DC-to-DC converter with a linear control characteristic of output voltage to switching duty cycle and an optimal reset of the transformer core. U.S. Pat. No. 5,198,969, issued to Redl et al, discloses a phase-shifted full bridge DC-to-DC converter capable of reducing switching losses of transistors and rectifier diodes. Further, U.S. Pat. No. 4,864,479, U.S. Pat. No. 5,946,200 and U.S. Pat. No. 6,504,739 all disclose full bridge DC-to-DC converters capable of reducing switching losses. However, the above mentioned soft switching full-bridge converters have a larger pulsating input current ripple inherent problem. In addition to the dv/dt phenomenon, the pulsating input current ripples, di/dt, causes the power converters having EMI problems. The lower the pulsating current ripples, the lower the EMI intensity to be dealt with. Consequently, a smaller EMI filter can be used to meet the EMI regulations. Moreover, a lower RMS value of the input current can be also achieved resulting in improving the converter efficiency. Therefore, low input current ripple power converter has additional advantage and becomes one of the design criteria of concern.
Thus, there is a need for a high frequency soft switching power converter with a lower input current ripple and high conversion efficiency.